mgcp_protocol: forward declare mgcp_endpoint

The function mgcp_rtp_end_config() takes an mgcp_endpoint as
parameter. The header file does not declare the mgcp_endp struct
and it also does not include mgcp_endp.h because that also would
cause problems. Since the endp parameter in mgcp_rtp_end_config()
is only a pointer we can simply forward declare it.

This patch will currently not change anythig, but it will prevent
compiler warnings when we remove the endp pointer from
struct mgcp_parse_data in a follow up patch

Related: SYS#5535
Change-Id: I07a4d6f9d5334b1f4cf4b262482b8a67b1384398

include/osmocom/mgcp/mgcp_protocol.h

@@ -20,6 +20,7 @@ char *get_lco_identifier(const char *options);
 int check_local_cx_options(void *ctx, const char *options);
 
 struct mgcp_rtp_end;
+struct mgcp_endpoint;
 void mgcp_rtp_end_config(struct mgcp_endpoint *endp, int expect_ssrc_change,
 			 struct mgcp_rtp_end *rtp);